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RFC 3949

Draft STD
Pages: 84
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File Format for Internet Fax

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Network Working Group                                         R. Buckley
Request for Comments: 3949                                    D. Venable
Obsoletes: 2301                                        Xerox Corporation
Category: Standards Track                                    L. McIntyre
                                                              Consultant
                                                              G. Parsons
                                                         Nortel Networks
                                                             J. Rafferty
                                                              Brooktrout
                                                           February 2005


                      File Format for Internet Fax

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document is a revised version of RFC 2301.  The revisions,
   summarized in the list attached as Annex B, are based on discussions
   and suggestions for improvements that have been made since RFC 2301
   was issued in March 1998, and on the results of independent
   implementations and interoperability testing.

   This RFC 2301 revision describes the Tag Image File Format (TIFF)
   representation of image data specified by the International
   Telecommunication Union (ITU-T) Recommendations for black-and-white
   and color facsimile.  This file format specification is commonly
   known as TIFF for Fax eXtended (TIFF-FX).  It formally defines
   minimal, extended, and lossless Joint Bi-level Image experts Group
   (JBIG) profiles (Profiles S, F, J) for black-and-white fax and base
   JPEG, lossless JBIG, and Mixed Raster Content profiles (Profiles C,
   L, M) for color and grayscale fax.  These profiles correspond to the
   content of the applicable ITU-T Recommendations.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
       1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . .  5
       1.2. Approach. . . . . . . . . . . . . . . . . . . . . . . . .  5
       1.3. Overview of this Document . . . . . . . . . . . . . . . .  5
   2.  TIFF and Fax . . . . . . . . . . . . . . . . . . . . . . . . .  7
       2.1. TIFF Overview . . . . . . . . . . . . . . . . . . . . . .  7
            2.1.1. File Structure . . . . . . . . . . . . . . . . . .  8
            2.1.2. Image Structure. . . . . . . . . . . . . . . . . . 10
            2.1.3. TIFF File Structure for Fax Applications . . . . . 11
       2.2. TIFF Fields for All Fax Applications. . . . . . . . . . . 12
            2.2.1. TIFF Fields required for all fax profiles. . . . . 13
            2.2.2. Additional TIFF Fields required for all fax
                   profiles . . . . . . . . . . . . . . . . . . . . . 14
            2.2.3. TIFF Fields recommended for all fax profiles . . . 17
            2.2.4. New TIFF Fields recommended for fax profiles . . . 18
   3.  Profile S: Minimal Black-and-White Fax Profile . . . . . . . . 20
       3.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . 20
       3.2. Required TIFF Fields. . . . . . . . . . . . . . . . . . . 21
            3.2.1. Baseline Fields. . . . . . . . . . . . . . . . . . 21
            3.2.2. Extension Fields . . . . . . . . . . . . . . . . . 23
            3.2.3. New Fields . . . . . . . . . . . . . . . . . . . . 23
       3.3. Recommended TIFF Fields . . . . . . . . . . . . . . . . . 23
       3.4. End of Line (EOL) and Return to Control (RTC) . . . . . . 23
            3.4.1. RTC Exclusion. . . . . . . . . . . . . . . . . . . 24
       3.5. File Structure. . . . . . . . . . . . . . . . . . . . . . 24
       3.6. Profile S: Minimal Black-and-White Profile Summary. . . . 26
   4.  Profile F: Extended Black-and-White Fax Profile. . . . . . . . 27
       4.1. TIFF-F Overview . . . . . . . . . . . . . . . . . . . . . 28
       4.2. Required TIFF Fields. . . . . . . . . . . . . . . . . . . 29
            4.2.1. Baseline Fields. . . . . . . . . . . . . . . . . . 29
            4.2.2. Extension Fields . . . . . . . . . . . . . . . . . 32
            4.2.3. New Fields . . . . . . . . . . . . . . . . . . . . 32
       4.3. Recommended TIFF Fields . . . . . . . . . . . . . . . . . 32
            4.3.1. Baseline Fields. . . . . . . . . . . . . . . . . . 32
            4.3.2. Extension Fields . . . . . . . . . . . . . . . . . 33
            4.3.3. New Fields . . . . . . . . . . . . . . . . . . . . 33
       4.4. Technical Implementation Issues . . . . . . . . . . . . . 34
            4.4.1. Strips . . . . . . . . . . . . . . . . . . . . . . 34
            4.4.2. Bit Order. . . . . . . . . . . . . . . . . . . . . 34
            4.4.3. Multi-Page . . . . . . . . . . . . . . . . . . . . 35
            4.4.4. Compression. . . . . . . . . . . . . . . . . . . . 35
            4.4.5. Example Use of Page-quality Fields . . . . . . . . 36
            4.4.6. Practical Guidelines for Writing and Reading
                   Multi-Page TIFF-F Files. . . . . . . . . . . . . . 36
            4.4.7. Use of TIFF-F for Streaming Applications . . . . . 38

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       4.5. Implementation Warnings . . . . . . . . . . . . . . . . . 38
            4.5.1. Uncompressed Data. . . . . . . . . . . . . . . . . 38
            4.5.2. Encoding and Resolution. . . . . . . . . . . . . . 38
            4.5.3. EOL byte-aligned . . . . . . . . . . . . . . . . . 39
            4.5.4. EOL. . . . . . . . . . . . . . . . . . . . . . . . 40
            4.5.5. RTC Exclusion. . . . . . . . . . . . . . . . . . . 40
            4.5.6. Use of EOFB for T.6 Compressed Images. . . . . . . 40
       4.6. Example Use of TIFF-F . . . . . . . . . . . . . . . . . . 40
       4.7. Profile F: Extended Black-and-white Fax Profile Summary . 41
   5.  Profile J: Lossless JBIG Black-and-White Fax Profile . . . . . 43
       5.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . 43
       5.2. Required TIFF Fields. . . . . . . . . . . . . . . . . . . 44
            5.2.1. Baseline Fields. . . . . . . . . . . . . . . . . . 44
            5.2.2. Extension Fields . . . . . . . . . . . . . . . . . 44
            5.2.3. New Fields . . . . . . . . . . . . . . . . . . . . 44
       5.3. Recommended TIFF Fields . . . . . . . . . . . . . . . . . 45
       5.4. Profile J: Lossless JBIG Black-and-White Profile Summary. 45
   6.  Profile C: Base Color Fax Profile. . . . . . . . . . . . . . . 47
       6.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . 47
       6.2. Required TIFF Fields. . . . . . . . . . . . . . . . . . . 47
            6.2.1. Baseline Fields. . . . . . . . . . . . . . . . . . 47
            6.2.2. Extension Fields . . . . . . . . . . . . . . . . . 49
            6.2.3. New Fields . . . . . . . . . . . . . . . . . . . . 50
       6.3. Recommended TIFF Fields . . . . . . . . . . . . . . . . . 52
       6.4. Profile C: Base Color Fax Profile Summary . . . . . . . . 52
   7.  Profile L: Lossless Color Profile. . . . . . . . . . . . . . . 54
       7.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . 54
            7.1.1. Color Encoding . . . . . . . . . . . . . . . . . . 54
            7.1.2. JBIG Compression . . . . . . . . . . . . . . . . . 55
       7.2. Required TIFF Fields. . . . . . . . . . . . . . . . . . . 55
            7.2.1. Baseline Fields. . . . . . . . . . . . . . . . . . 56
            7.2.2. Extension Fields . . . . . . . . . . . . . . . . . 57
            7.2.3. New Fields . . . . . . . . . . . . . . . . . . . . 57
       7.3. Recommended TIFF Fields . . . . . . . . . . . . . . . . . 57
       7.4. Profile L: Lossless Color Fax Profile Summary . . . . . . 58
   8.  Profile M: Mixed Raster Content Profile. . . . . . . . . . . . 60
       8.1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . 60
            8.1.1. MRC 3-layer model. . . . . . . . . . . . . . . . . 60
            8.1.2. A TIFF Representation for the MRC 3-layer model. . 62
       8.2. Required TIFF Fields. . . . . . . . . . . . . . . . . . . 64
            8.2.1. Baseline Fields. . . . . . . . . . . . . . . . . . 64
            8.2.2. Extension Fields . . . . . . . . . . . . . . . . . 66
            8.2.3. New Fields . . . . . . . . . . . . . . . . . . . . 67
       8.3. Recommended TIFF Fields . . . . . . . . . . . . . . . . . 69
       8.4. Rules and Requirements for Images . . . . . . . . . . . . 69
       8.5. Profile M: MRC Fax Profile Summary. . . . . . . . . . . . 71
   9.  MIME content-types image/tiff and image/tiff-fx. . . . . . . . 74
   10. Security Considerations. . . . . . . . . . . . . . . . . . . . 74

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   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 74
       11.1. Normative References . . . . . . . . . . . . . . . . . . 74
       11.2. Informative References . . . . . . . . . . . . . . . . . 76
   Annex A: Summary of TIFF Fields for Internet Fax . . . . . . . . . 77
   Annex B: List of technical edits to RFC 2301 . . . . . . . . . . . 81
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 82
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 84

1.  Introduction

   This document describes the use of TIFF (Tag Image File Format) to
   represent the data content and structure generated by the current
   suite of ITU-T Recommendations for Group 3 facsimile.  These
   recommendations and the TIFF fields described here support the
   following facsimile profiles:

   S:  Minimal black-and-white profile, using binary MH compression
       [T.4]
   F:  Extended black-and-white profile, using binary MH, MR, and MMR
       compression [T.4, T.6]
   J:  Lossless JBIG black-and-white profile, with JBIG compression
       [T.85, T.82]
   C:  Lossy color and grayscale profile, using JPEG compression [T.42,
       T.81]
   L:  Lossless color and grayscale profile, using JBIG compression
       [T.43, T.82]
   M:  Mixed raster content profile [T.44], using a combination of
       existing compression methods

   Each profile corresponds to the content of ITU-T Recommendations
   shown and is a subset of the full TIFF for facsimile specification.

   Profile S describes a minimal interchange set of fields, which will
   guarantee that, at least, binary black-and-white images will be
   supported.  Implementations are required to support this minimal
   interchange set of fields.

   With the intent of specifying a file format for Internet fax, this
   document

   1. specifies the structure of TIFF files for facsimile data,
   2. defines ITU fax-compatible values for existing TIFF fields, and
   3. defines new TIFF fields and values required for compatibility with
      ITU color fax.

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   This specification of TIFF for facsimile is known as TIFF-FX (TIFF
   for Fax eXtended).  References to the format described by this
   specification should always use the term "TIFF-FX", and some profiles
   in this specification may not be interpreted correctly by other TIFF
   applications.

1.1.  Scope

   This document defines a TIFF-based file format specification for
   enabling standardized messaging-based fax over the Internet.  It
   specifies the TIFF fields and field values required for compatibility
   with the existing ITU-T Recommendations for Group 3 black-and-white,
   grayscale, and color facsimile.  TIFF has historically been used for
   handling fax image files in applications such as store-and-forward
   messaging.  Implementations that support this file format
   specification for import/export may elect to support it as a native
   format.  This document recommends a TIFF file structure compatible
   with low-memory and page-level streaming implementations.

   Unless otherwise noted, the current TIFF specification [TIFF] and
   selected TIFF Technical Notes [TTN1, TTN2] are the primary references
   for describing TIFF and defining TIFF fields.  This document is the
   primary reference for defining TIFF field values for fax
   applications.

1.2.  Approach

   The basic approach to using TIFF for facsimile data is to insert the
   compressed fax image data into a TIFF file and use TIFF fields to
   encode the parameters that describe the image data.  These fields
   will have values that comply with the ITU-T Recommendations.

   This approach takes advantage of TIFF features and structures that
   bridge the data formats and performance requirements of both legacy
   fax machines and host-based fax applications.  TIFF constructs for
   pages, images, and strips allow a TIFF file to preserve the fax data
   stream structure and the performance advantages that come with it.  A
   TIFF-based approach also builds on an established base of users and
   implementors and ensures backward compatibility with existing TIFF-
   based IETF proposals and work in progress for Internet fax.

1.3.  Overview of this Document

   Section 2 gives an overview of TIFF.  Section 2.1 describes the
   structure of TIFF files, including general guidelines for structuring
   multi-page TIFF files.  Section 2.2 lists the TIFF fields that are
   required or recommended for all fax profiles.  The TIFF fields used

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   only by specific fax profiles are described in Sections 3 - 8, which
   describe the individual fax profiles.  These sections also specify
   the ITU-compatible field values (image parameters) for each profile.

   The full set of permitted fields of TIFF for facsimile are included
   in the current TIFF specification, Section 2 of this document, and
   the sections on specific profiles of facsimile operation.  This
   document defines profiles of TIFF for facsimile, where a profile is a
   subset of the full set of permitted fields and field values of TIFF
   for facsimile.

   Section 3 defines the minimal black-and-white facsimile profile
   (Profile S), which is required in all implementations.  Section 4
   defines the extended black-and-white fax profile (Profile F), which
   provides a standard definition of TIFF-F.  Section 5 describes the
   lossless black-and-white profile using JBIG compression (Profile J).

   Section 6 defines the base color profile, required in all color
   implementations, for the lossy JPEG representation of color and
   grayscale facsimile data (Profile C).  Section 7 defines the lossless
   JBIG color and grayscale facsimile profile (Profile L), and Section 8
   defines the Mixed Raster Content facsimile profile (Profile M).  Each
   of these sections concludes with a table summarizing the required and
   recommended fields for each profile and the values they can have.

   Section 9 refers to the MIME content types used in connection with
   TIFF for facsimile.  Sections 10 and 11 give Security Considerations
   and References, followed by Authors' Addresses and the Copyright
   Notice.  Annex A gives a summary of the TIFF fields used or defined
   in this document and provides a convenient reference for
   implementors.

   To implement only the minimal interchange black-and-white set of
   fields and values (Profile S), one need read only Sections 1, 2, 3,
   9, and 10.

   The following tree diagram shows the relationship among profiles and
   between profiles and coding methods.

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                        S (MH)
                       / \
               B&W    /   \   Color
          ------------     ----------
         /      \                    \
        /        F (MH, MR, MMR)      C (JPEG)
       /                             / \
      J (JBIG)                   ----   \
                                /        \
                               L (JBIG)   \
                                           \
                                            M (MRC)

   A profile is based on a collection of ITU-T facsimile coding methods.
   For example, Profile S, the minimal profile, is based on Modified
   Huffman (MH) compression, which is defined in ITU-T Rec. T.4.
   Profile F specifies Modified Huffman (MH), Modified READ (MR), and
   Modified Modified READ (MMR) compressions, which are defined in ITU-T
   Rec. T.4 and T.6.

   All implementations of TIFF for facsimile MUST implement Profile S,
   which is the root node of the tree.  All color implementations of
   TIFF for facsimile MUST implement Profile C.  The implementation of a
   particular profile MUST also implement those profiles on the path
   that connect it to the root node, and MAY optionally implement
   profiles not on the path connecting it to the root node.  For
   example, an implementation of Profile M must also implement Profiles
   C and S and may optionally implement Profile F, J, or L.  For another
   example, an implementation of Profile C must also implement Profile S
   and may optionally implement Profile F or J.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", " NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [REQ].

2.  TIFF and Fax

2.1.  TIFF Overview

   TIFF provides a means for describing, storing, and interchanging
   raster image data.  A primary goal of TIFF is to provide a rich
   environment within which applications can exchange image data.  The
   current TIFF specification [TIFF] defines a commonly used core set of
   TIFF fields known as Baseline TIFF.  The current specification, the
   set of Pagemaker TIFF Technical Notes [TTN1], and TIFF Technical Note
   2 [TTN2] define several TIFF extensions.  The TIFF-based
   specification for fax applications uses a subset of Baseline TIFF

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   fields, with selected extensions, as described in this document.  In
   a few cases, this document defines new TIFF fields specifically for
   fax applications.

2.1.1.  File Structure

   TIFF is designed for raster images, which makes it a good match for
   facsimile documents, which are multi-page raster images.  Each raster
   image consists of a number of rows or scanlines, each of which has
   the same number of pixels, the unit of sampling.  Each pixel has at
   least one sample or component (exactly one for black-and-white
   images).

   A TIFF file begins with an 8-byte image file header.  The first two
   bytes describe the byte order used within the file.  Legal values are
   "II" (0x4949) when bytes are ordered from least to most significant
   (little-endian), and "MM" (0x4D4D), when bytes are ordered from most
   to least significant (big-endian) within a 16- or 32-bit integer.
   Either byte order can be used, except in the case of the minimal
   black-and-white profile, which SHALL use value "II".  The next two
   bytes contain the value 42, which identifies the file as a TIFF file
   and is ordered according to the value in the first two bytes of the
   header.  The last four bytes give the offset that points to the first
   image file directory (IFD).  This and all other offsets in a TIFF
   file are with respect to the beginning of the TIFF file.  An IFD can
   be at any location in the file after the header but must begin on a
   word boundary.

   An IFD is a sequence of tagged fields, sorted in ascending order by
   tag value.  An IFD consists of a 2-byte count of the number of
   fields, a sequence of field entries, and a 4-byte offset to the next
   IFD.  The fields contain information about the image and pointers to
   the image data.  Each separate raster image in the file is
   represented by an IFD.

   Each field entry in an IFD has 12 bytes and consists of a 2-byte Tag,
   2 bytes identifying the field type (e.g., short, long, rational,
   ASCII), 4 bytes giving the count (number of values or offsets), and 4
   bytes containing either the offset to a field value stored outside
   the IFD or, based on the type and count, the field value itself.
   Resolution and metadata such as dates, names, and descriptions are
   examples of "long" field values that do not fit in 4 bytes and
   therefore use offsets in the field entry.  Details are given in the
   TIFF specification [TIFF].

   A TIFF file can contain more than one IFD, where each IFD is a
   subfile whose type is given in the NewSubfileType field.  Multiple
   IFDs can be organized either as a linked list, with the last entry in

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   each IFD pointing to the next IFD (the pointer in the last IFD is 0),
   or as a tree, using the SubIFDs field in the primary IFD [TTN1].  The
   SubIFDs field contains an array of pointers to child IFDs of the
   primary IFD.

   Child IFDs describe related images, such as reduced resolution
   versions of the primary IFD image.  The same IFD can point both to a
   next IFD and to child IFDs, and child IFDs can themselves point to
   other IFDs.

   All fax profiles represent a multi-page fax image as a linked list of
   IFDs, with a NewSubfileType field containing a bit that identifies
   the IFD as one page of a multi-page document.  Each IFD has a
   PageNumber field, identifying the page number in ascending order,
   starting at 0 for the first page.  Although a Baseline TIFF reader is
   not required to read any IFDs beyond the first, an implementation
   that reads the files that comply with this specification SHALL read
   multiple IFDs.  Only the Mixed Raster Content fax profile, described
   in Section 8, requires the use of child IFDs.

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   The following figure illustrates the structure of a multi-page TIFF
   file.

              +-----------------------+
              |         Header        |------------+
              +-----------------------+            | First IFD
              |      IFD (page 0)     |<-----------+ Offset
          +---|                       |------------+
    Value |   +-----------------------+            |
   Offset +-->|      Long Values      |--+         |
              +-----------------------|  | Strip   |
              |       Image Data      |<-+ Offset  |
              |     strip 1 page 0    |  |         |
              +-----------------------+  |         |
              |           :           |  :         |
                                                   |
              +-----------------------+            | Next IFD
              |      IFD (page 1)     |<-----------+ Offset
          +---|                       |------------+
    Value |   +-----------------------+            |
   Offset +-->|      Long Values      |--+         |
              +-----------------------|  | Strip   |
              |       Image Data      |<-+ Offset  |
              |     strip 1 page 1    |  |         |
              +-----------------------+  |         |
              |     strip 2 page 1    |<-+         |
              +-----------------------+  |         |
              |          :            |  :         |
                                                   |
              +-----------------------+            | Next IFD
              |      IFD (page 2)     |<-----------+ Offset
              |          :            |

2.1.2.  Image Structure

   An IFD stores an image as one or more strips, as shown in the
   preceding figure.  A strip consists of 1 or more scanlines (rows) of
   raster image data in compressed form.  An image may be stored in a
   single strip or may be divided into several strips, which would
   require less memory to buffer.  (Baseline TIFF recommends about 8k
   bytes per strip, but existing fax usage is typically one strip per
   image.)

   Each IFD requires three strip-related fields: StripOffsets,
   RowsPerStrip, and StripByteCounts.  The StripOffsets field is an
   array of pointers to the strip or strips that contain the actual
   image data.  The StripByteCounts field gives the number of bytes in
   each strip after compression.  TIFF requires that each strip, except

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   the last, contain the same number of scanlines, which is given in the
   RowsPerStrip field.  This document introduces the new StripRowCounts
   field that allows a variable number of scanlines per strip, which is
   required by the Mixed Raster Content fax profile (Section 8).

   Image data is stored as uninterpreted, compressed image data streams
   within a strip.  The formats of these streams follow the ITU-T
   Recommendations.  The Compression field in the IFD indicates the type
   of compression, and other TIFF fields in the IFD describe image
   attributes such as color encoding and spatial resolution.
   Compression parameters are stored in the compressed data stream
   rather than in TIFF fields.  This makes the TIFF representation and
   compressed data format specification independent of each another.
   This approach, modeled on [TTN2], allows TIFF to add new compression
   schemes gracefully as they become available.

   Some attributes can be specified both in the compressed data stream
   and within a TIFF field.  It is possible that the two values will
   differ.  When this happens for values required to interpret the data
   stream, the values in the data stream take precedence.  For
   informational values that are not required to interpret the data
   stream, such as author name, then the TIFF field value takes
   precedence.

2.1.3.  TIFF File Structure for Fax Applications

   The TIFF specification has a very flexible file structure that does
   not specify the ordering of IFDs, field values, and image data in a
   file.  Individual applications may require or recommend an ordering.

   This specification recommends that when using a TIFF file for
   facsimile, a multi-page fax document SHOULD be represented as a
   linked list of IFDs.  It also recommends that a TIFF file for
   facsimile SHOULD order pages in a TIFF file in the same way that they
   are ordered in a fax data stream.  In a TIFF file, a page consists of
   several elements: one or more IFDs (including subIFDs), long field
   values that are stored outside the IFDs, and image data (in one or
   more strips).

   The minimal black-and-white profile (Profile S) specifies a required
   ordering of pages and elements within a page (Section 3.5).  The
   extended black-and-white profile (Profile F) provides guidelines for
   ordering pages and page elements (Section 4.4.6).  Other profiles
   SHOULD follow these guidelines.  This recommendation is intended to
   simplify the implementation of TIFF writers and readers in fax
   applications and the conversion between TIFF file and fax data stream
   representations.  However, for interchange robustness, readers SHOULD

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   be prepared to read TIFF files whose structure is consistent with
   [TIFF], which supports a more flexible file structure than is
   recommended here.

   This specification introduces an optional new GlobalParametersIFD
   field, defined in Section 2.2.4.  This field has type IFD and
   indicates parameters describing the fax session.  While it is often
   possible to obtain these parameters by scanning the file, it is
   convenient to make them available together in one place for fast and
   easy access.  If the GlobalParametersIFD occurs in a TIFF file, it
   SHOULD be located in the first IFD, immediately following the 8-byte
   image file header.

2.2.  TIFF Fields for All Fax Applications

   The TIFF specification [TIFF] is organized as a baseline set and
   several extensions, including technical notes [TTN1, TTN2] that will
   be incorporated in the next release of TIFF.  The baseline and
   extensions have required and optional fields.

   Facsimile applications require (and recommend) a mixture of baseline
   and extensions fields, as well as some new fields that are not part
   of the TIFF specification and that are defined in this document.
   This sub-section lists the fields that are required or recommended
   for all profiles.  In particular, Section 2.2.1 lists the fields that
   are required by all profiles and that have values that do not depend
   on the profile.  Section 2.2.2 lists the fields that are required by
   all profiles and that have values that do depend on the profile.
   Section 2.2.3 lists the fields that are recommended for all profiles.
   Fields required or recommended by some but not all profiles are given
   in the section (Section 3 - 8) that describes that profile.  The
   sections for each fax profile have subsections for required and
   recommended fields; each subsection organizes the fields according to
   whether they are baseline, extension or new.

   The fields required for facsimile have only a few legal values,
   specified in the ITU-T Recommendations.  Of these legal values, some
   are required and some are optional, just as they are required
   (mandatory) or optional in fax implementations that conform to the
   ITU-T Recommendations.  The required and optional values are noted in
   the sections on the different fax profiles.

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   This section describes the fields required or recommended by all fax
   profiles.  The pattern for the description of TIFF fields in this
   document is as follows:

   FieldName(TagValueInDecimal) = allowable values.
   TYPE

      WhetherRequiredByTIFForTIFFforFAX
      Count = (omitted if =1) = (if not in current spec but available)
      Explanation of the field, how it's used, and the values it can
      have.  Default value, if any, as specified in [TIFF].

   When a field's default value is the desired value, that field may be
   omitted from the relevant IFD unless specifically required by the
   text of this specification.

2.2.1.  TIFF fields required for all fax profiles

   The TIFF fields listed in this section SHALL be used by all fax
   profiles but have field values that are not specified by the ITU
   standards, i.e., the fields do not depend on the profile.  The next
   subsection lists the fields that SHALL be used by all fax profiles,
   but which do have values specified by the ITU-specified or profile-
   specific values.  Fields that SHALL be used by some but not all
   profiles are given in the Sections (3 - 8) that describe the profiles
   that use them.

   ImageLength(257)
   SHORT or LONG
      RequiredByTIFFBaseline
      Total number of scanlines in image.
      No default, must be specified.

   PageNumber(297)
   SHORT
      RequiredByTIFFforFAX, TIFFExtension
      Count = 2
      The first number represents the page number (0 for the first
      page); the second number is the total number of pages in the
      document.  If the second value is 0, then the total page count is
      not available.
      No default, must be specified

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   RowsPerStrip(278)
   SHORT or LONG
      RequiredByTIFFBaseline
      The number of scanlines per TIFF strip, except for the last strip.
      For a single strip image, this is the same as the value of the
      ImageLength field.
      Default = 2**32 - 1 (meaning all scanlines in one strip).

   StripByteCounts(279)
   SHORT or LONG
      RequiredByTIFFBaseline
      Count = number of strips
      For each strip, the number of bytes in that strip after
      compression.
      No default, must be specified.

   StripOffsets(273)
   SHORT or LONG
      RequiredByTIFFBaseline
      Count = number of strips
      For each strip, the byte offset from the beginning of the file to
      the start of that strip.
      No default, must be specified.

2.2.2.  Additional TIFF fields required for all fax profiles

   The TIFF fields listed in this section SHALL be used by all fax
   profiles, but the values associated with them depend on the profile
   being described and the associated ITU Recommendations.  Therefore,
   only the fields are defined here; the values applicable to a
   particular fax profile are described in Sections 3 - 8.  Fields that
   SHALL be used by some but not all profiles are given in the section
   (3 - 8) describing the profile that uses them.

   BitsPerSample(258)
   SHORT
      RequiredByTIFFBaseline
      Number of bits per image sample.
      Default = 1 (field may be omitted if this is the value).

   Compression(259)
   SHORT
      RequiredByTIFFBaseline
      Compression method used for image data.
      Default = 1 (no compression, so may not be omitted for FAX).

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   FillOrder(266)
   SHORT
      RequiredByTIFFforFax
      The default bit order in Baseline TIFF per [TIFF] is indicated by
      FillOrder=1, where bits are not reversed before being stored.
      However, TIFF for Fax typically uses the setting of FillOrder=2,
      where the bit order within bytes is reversed before storage (i.e.,
      bits are stored with the Least Significant Bit first).
      Default = 1 (field may be omitted if this is the value)
      Facsimile data appears on the phone line in bit-reversed order
      relative to its description in the relevant ITU compression
      Recommendation.  Therefore, a wide majority of facsimile
      implementations choose this natural order for storage.

      Nevertheless, all readers conforming to this specification must be
      able to read data in both bit orders, except in the case of
      Profile S, which only requires support for FillOrder=2 (Least
      Significant Bit first).

   ImageWidth(256)
   SHORT or LONG
      RequiredByTIFFBaseline
      The number of pixels (columns) per scanline (row) of the image
      No default, must be specified.

   NewSubFileType(254)
   LONG
      RequiredByTIFFforFAX
      A general indication of the kind of data contained in this IFD Bit
      1 is 1 if the image is a single page of a multi-page document.
      Default = 0 (no subfile bits on, so may not be omitted for FAX).

   PhotometricInterpretation(262)
   SHORT
      RequiredByTIFFBaseline
      The color space of the image data.
      No default, must be specified.

   ResolutionUnit(296)
   SHORT
      RequiredByTIFFBaseline The unit of measure for resolution. 2 =
      inch, 3 = centimeter; Default = 2 (field may be omitted if this is
      the value)

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   SamplesPerPixel(277)
   SHORT
      RequiredByTIFFBaseline
      The number of color components per pixel; SamplesPerPixel is 1 for
      a black-and-white, grayscale or indexed (palette) image.  Default
      = 1 (field may be omitted if this is the value).

   XResolution(282)
   RATIONAL
      RequiredByTIFFBaseline
      The horizontal resolution of the image in pixels per resolution
      unit.  The ITU-T Recommendations for facsimile specify a small
      number of horizontal resolutions: 100, 200, 300, 400 pixels per
      inch, and 80, 160 pixels per centimeter (or 204, 408 pixels per
      inch).  The allowed XResolution values for each profile are given
      in the section defining that profile.  Per [T.4], it is
      permissible for applications to treat the following XResolution
      values as being equivalent: <204, 200> and <400,408> in
      pixels/inch.  These equivalencies were allowed by [T.4] to permit
      conversions between inch and metric based facsimile terminals.  To
      ensure interoperability, if an application accepts any member of
      the pairs then T.4 requires it to accept both (e.g., accept 204 if
      200 pixels per inch is accepted).  TIFF for Facsimile Writers
      SHOULD express XResolution in inch-based units, for consistency
      with historical practice and to maximize interoperability.  See
      the table below for information on how to convert from an ITU-T
      metric value to its inch-based equivalent resolution.
      No default, must be specified

   YResolution(283)
   RATIONAL
      RequiredByTIFFBaseline
      The vertical resolution of the image in pixels per resolution
      unit.  The ITU-T Recommendations for facsimile specify a small
      number of vertical resolutions: 100, 200, 300, 400 pixels per
      inch, and 38.5, 77, 154 pixels per centimeter (or 98, 196, 391
      pixels per inch).  The allowed YResolution values for each profile
      are given in the section defining that profile.  Per [T.4], it is
      permissible for applications to treat the following YResolution
      values as being equivalent: <98, 100>, <196, 200>, and <391, 400>
      in pixels/inch.  These equivalencies were allowed by [T.4] to
      permit conversions between inch- and metric-based facsimile
      terminals.  To insure interoperability, if an application accepts
      any member of the pairs, then T.4 requires it to accept both
      (e.g., accept 98 if 100 pixels per inch is accepted).  TIFF for
      Facsimile Writers SHOULD express YResolution in inch-based units,
      for consistency with historical practice and to maximize

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      interoperability.  See the table below for information on
      converting from the metric value to its inch based equivalent
      resolution.
      No default, must be specified.

   +-----------------------------+-----------------------------+
   |         XResolution         |         YResolution         |
   +--------------+--------------+--------------+--------------+
   |ResolutionUnit|ResolutionUnit|ResolutionUnit|ResolutionUnit|
   |  =2 (inch)   |   =3 (cm)    |  =2 (inch)   |   =3 (cm)    |
   +--------------+--------------+--------------+--------------+
   |     100      |              |     100      |              |
   +--------------+--------------+--------------+--------------+
   |     204      |      80      |      98      |     38.5     |
   |     200      |              |     100      |              |
   +--------------+--------------+--------------+--------------+
   |     204      |      80      |     196      |      77      |
   |     200      |              |     200      |              |
   +--------------+--------------+--------------+--------------+
   |     204      |      80      |     391      |     154      |
   +--------------+--------------+--------------+--------------+
   |     300      |              |     300      |              |
   +--------------+--------------+--------------+--------------+
   |     408      |     160      |     391      |     154      |
   |     400      |              |     400      |              |
   +--------------+--------------+--------------+--------------+

2.2.3.  TIFF fields recommended for all fax profiles

   The TIFF fields listed in this section MAY be used by all fax
   profiles.  However, Profile S writers (the minimal fax profile
   described in Section 3) SHOULD NOT use these fields.  Recommended
   fields that are profile-specific are described in Sections 3 - 8.

   DateTime(306)
   ASCII
      OptionalInTIFFBaseline
      Date/time of image creation in 24-hour format
      "YYYY:MM:DD HH:MM:SS".  No default.

   DocumentName(269)
   ASCII
      OptionalInTIFFExtension(DocumentStorageAndRetrieval)
      The name of the scanned document.  This is a TIFF extension field,
      not a Baseline TIFF field.  No default.

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   ImageDescription(270)
   ASCII
      OptionalInTIFFBaseline
      A string describing the contents of the image.
      No default.

   Orientation(274) = 1 - 8.
   SHORT
      OptionalinTIFFBaseline 1: 0th row represents the visual top of the
      image; the 0th column represents the visual left side of the
      image.  See the current TIFF spec [TIFF] for further values;
      Baseline TIFF only requires value=1.  Default = 1.
      Note: It is recommended that a writer that is aware of the
      orientation include this field to give a positive indication of
      the orientation, even if the value is the default.  Writers should
      not generate mirror images, because many readers will not properly
      reverse the image before display or print.

   Software(305)
   ASCII
      OptionalInTIFFBaseline
      The name and release number of the software package that
      created the image.
      No default.

2.2.4.  New TIFF fields recommended for fax profiles

   The new TIFF fields listed in this section MAY be used by all fax
   profiles.  However, Profile S writes (the minimal fax profile
   described in Section 3) SHOULD NOT use these fields.  In addition,
   support for these new TIFF fields has not been included in historical
   TIFF-F readers described in Section 4 and [TIFF-F].  These fields
   describe "global" parameters of the fax session that created the
   image data.  They are optional, not part of the current TIFF
   specification, and are defined in this document.

   The first new field, GlobalParametersIFD, is an IFD that contains
   global parameters and is located in a Primary IFD.

   GlobalParametersIFD (400) IFD or LONG

      An IFD containing global parameters.  It is recommended that a
      TIFF writer place this field in the first IFD, where a TIFF reader
      would find it quickly.

      Each field in the GlobalParametersIFD is a TIFF field that is
      legal in any IFD.  Required baseline fields should not be located
      in the GlobalParametersIFD but should be in each image IFD.  If a

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      conflict exists between fields in the GlobalParametersIFD and in
      the image IFDs, then the data in the image IFD shall prevail.

      Among the GlobalParametersIFD entries is a new ProfileType field
      that generally describes information in this IFD and in the TIFF
      file.

   ProfileType(401)
   LONG
      The type of image data stored in this IFD.
      0 = Unspecified
      1 = Group 3 fax
      No default

   The following new global fields are defined in this document as IFD
   entries for use with fax applications.

   FaxProfile(402) = 0 - 6.
   BYTE
      The profile that applies to this file; a profile is subset of the
      full set of permitted fields and field values of TIFF for
      facsimile.  The currently defined values are:
      0: does not conform to a profile defined for TIFF for facsimile
      1: minimal black & white lossless, Profile S
      2: extended black & white lossless, Profile F
      3: lossless JBIG black & white, Profile J
      4: lossy color and grayscale, Profile C
      5: lossless color and grayscale, Profile L
      6: Mixed Raster Content, Profile M

   CodingMethods(403)
   LONG
      This field indicates which coding methods are used in the file.  A
      value of 1 in a bit location indicates the corresponding coding
      method is used.  More than one bit set to 1 means more than one
      coding method is used in the file.
   Bit 0: unspecified compression
   Bit 1: 1-dimensional coding, ITU-T Rec. T.4 (MH - Modified Huffman)
   Bit 2: 2-dimensional coding, ITU-T Rec. T.4 (MR - Modified READ)
   Bit 3: 2-dimensional coding, ITU-T Rec. T.6 (MMR - Modified MR)
   Bit 4: ITU-T Rec. T.82 coding, using ITU-T Rec. T.85 (JBIG)
   Bit 5: ITU-T Rec. T.81 (Baseline JPEG)
   Bit 6: ITU-T Rec. T.82 coding, using ITU-T Rec. T.43 (JBIG color)
   Bits 7 - 31: reserved for future use

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   Note: There is a limit of 32 compression types to identify standard
   compression methods.

   VersionYear(404)
   BYTE
      Count: 4
      The year of the standard specified by the FaxProfile field, given
      as 4 characters, e.g., '1997'; used in lossy and lossless color
      profiles.

   ModeNumber (405)
   BYTE
      The mode of the standard specified by the FaxProfile field.  A
      value of 0 indicates Mode 1.0; used in Mixed Raster Content
      profile.



(page 20 continued on part 2)

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